[71.03] EUVE Observations of a Giant Flare from UX Arietis

A. K. Dupree, N. S. Brickhouse (Harvard-Smithsonian CfA)

The active RSCVn-type binary star UX Arietis (HD 21242) was fortuitously
caught in a high flaring state when the Extreme
Ultraviolet Explorer satellite began observations
on 19 Nov. 1995 (UT 10:45). The count
rate in the Deep Survey (DS) instrument that samples
a band comparable to the Short Wavelength
spectrograph (70Å\ to 140Åwas a factor
of ten higher than measured during a previous observing sequence
on 7--10 Nov. 1995. This event is not only
the brightest RSCVn flare observed by EUVE, but also marks the largest
relative increase in source brightness for a RSCVn star ever measured
by EUVE. Thanks to
the EUVE Target of Opportunity program, our
pre-planned observing time was extended to 25 November 1995 (UT 11:00),
enabling the decay of the flare to be monitored over
almost the complete orbital period of 6.44 days. During
the decay, two other weaker flares occurred as
detected by the DS instrument. UX Ari
had not returned to its pre-flare emission level by the
end of the observations, but the DS count
rate remained a factor of 3 above its
quiescent state. Our
study of previous euv spectra of UX Ari shows an emission
measure distribution dominated by a narrow enhancement
(a ``bump'')
at high temperatures (\sim 10^7K) similar to
that found in other short-period RSCVn systems.
The spectra (70Å\ -- 400Åof the flaring event
are dominated by emission lines of He II and highly ionized iron
(in particular Fe XX, XXIII, and XXIV)
which have substantially increased flux
as compared to the quiescent state. Emission measures and
densities can be derived from spectra obtained at various
times in order to follow the decay of the coronal
plasma.